In order to improve the quality and productivity of a temperature measurement target material such as a wheel, a steel pipe, a steel sheet, or a rail, it has become important to manage the temperature of the temperature measurement target material in a cooling process. When the surface temperature of the temperature measurement target material is measured by use of a radiation thermometer in a cooling process in a hot rolling line or a heat treatment-and-cooling line of the temperature measurement target material, for example, sometimes there may be steam or scattered cooling water between the temperature measurement target material and the radiation thermometer. Alternatively, a surface of the temperature measurement target material may be covered with a water film or be submerged. Under such an environment, thermal radiation light emitted from the temperature measurement target material may be absorbed into water vapor, steam, cooling water, or the like, or may be scattered, and accordingly, the measured temperature value may include an error or the measurement may fail.
Accordingly, in order to reduce errors in measuring the temperature (hereinafter also referred to as temperature measurement errors), generated by the above factors, and to enable accurate radiant temperature measuring, there have been proposed various methods of measuring a surface temperature of a steel material according to the related art. For example, Patent Document 1 proposes a method of measuring a surface temperature of a steel material by forming a water column between a radiation thermometer and the surface of the steel material by ejection of purge water from a nozzle toward the surface of the steel material, and by detecting the radiant energy of the thermal radiation light emitted from the steel material through the water column.
More specifically, with the temperature measuring method disclosed in Patent Document 1, a water column is formed between a radiation thermometer and a measurement target. The radiation thermometer is configured to measure a surface temperature of the measurement target on the basis of the radiant energy emitted from the measurement target. From all the radiant energy emitted from the measurement target, part of the radiant energy is absorbed into the water column. Thus, considering the absorption, while the radiant energy is corrected, the surface temperature of the measurement target is measured by use of the radiation thermometer. This method is characterized by setting the temperature of the water column to 60° C. or higher to form the water column.
With the method disclosed in Patent Document 1, since the water column is formed between the radiation thermometer and the measurement target, water vapor or scattered water is unlikely to enter a part where the water column is formed, and it is possible to reduce the temperature measurement errors caused by absorption or scattering of the radiant energy by water vapor or scattered water. Further, with the method disclosed in Patent Document 1, since the temperature of the water column is set to 60° C. or higher, a boiling film is likely to be formed on the surface of the measurement target in contact with the water column. Accordingly, it becomes possible to suppress a decrease in the surface temperature of the measurement target and to reduce cooling unevenness of the measurement target without damaging the representativeness of the measured temperature value, which is advantageous.
However, the method disclosed in Patent Document 1 has the following problems. A heater for increasing the temperature of the water column to 60° C. or higher is necessary. A high energy cost is also necessary to increase the temperature of the water. Further, because a thickness measuring apparatus for measuring the thickness of the water column (using an ultrasonic system, for example) is necessary, the dimension of the entire apparatus is large, and accordingly, it becomes difficult to install the apparatus in a narrow space such as a space between steel material carrying rolls. Furthermore, even when the thickness measuring apparatus is installed, the maintainability may be hindered because the attachment and detachment may be troublesome, and a trouble of the thickness measuring apparatus may degrade the stability and reliability of the measured temperature value.
In order to solve at least one of the above problems and the like of the method disclosed in Patent Document 1, the present inventors have proposed a method disclosed in Patent Document 2. Specifically, the method disclosed in Patent Document 2 is a method of measuring a surface temperature of a temperature measurement target steel material by detecting thermal radiation light emitted from the bottom surface of the temperature measurement target steel material by use of a radiation thermometer that is disposed to be opposed to the bottom of the temperature measurement target steel material through purge water ejected from a nozzle toward the bottom surface of the temperature measurement target steel material. With this method, on the basis of the position of a path line of the temperature measurement target steel material, all heads of the purge water are set within a predetermined range (claim 2 of Patent Document 2). Further, this method proposes to set the wavelength of the thermal radiation light detected by the radiation thermometer to 0.9 μm or shorter (claim 3 of Patent Document 2).
According to the above method disclosed in Patent Document 2, since all heads of the purge water are set within a predetermined range, collision pressure of the purge water on the bottom surface of the temperature measurement target steel material is suppressed, and even when the purge water is at room temperature, cooling can be suppressed. Accordingly, according to the method disclosed in Patent Document 2, it becomes possible to obtain the advantage that the high energy cost for increasing the temperature of the water, which is necessary in Patent Document 1, is unnecessary. Further, by setting the wavelength of the thermal radiation light detected by the radiation thermometer to 0.9 μm or shorter, it becomes possible to obtain the advantage that the thickness measuring apparatus for measuring the thickness of the water column is unnecessary.
However, according to the above method disclosed in Patent Document 2, in a case in which the temperature of the top surface, side surfaces, and the like of the temperature measurement target steel material is measured, the purge water collides with the temperature measurement target steel material, and accordingly, temperature measurement errors may be generated by cooling of the surface of the temperature measurement target steel material. Further, since the wavelength of the thermal radiation light detected by the radiation thermometer is set to 0.9 μm or shorter, the lower limit of the surface temperature of the steel material which can be subjected to radiant temperature measuring is approximately 500° C. Considering recent requirement for high quality of the steel material, it has become important to manage the surface temperature in a low temperature region of approximately 200° C. Accordingly, it is difficult to manage appropriate temperatures with the method by which only the surface temperature of approximately 500° C. or higher can be measured.
In addition, as a technique to measure the surface temperature of a temperature measurement target material, generally, the temperature is measured by pressing a temperature sensor of a thermocouple wire lightly on the temperature measurement target material. The temperature sensor of the thermocouple is fixed on a rear surface of a contact plate at the edge of a temperature measurement unit. However, since the temperature sensor of the thermocouple wire is pressed by the contact plate on the temperature measurement target material, in a cooling process of water cooling, water enters a space between the contact plate and the temperature measurement target material, and the temperature sensor of the thermocouple wire contacts the water. Accordingly, it becomes difficult to measure the surface temperature of the temperature measurement target material accurately.